Electronic musical instrument



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United States Patent 2,924,784 ELECTRONIC MUSICAL INSTRUMENT Richard H.Peterson, Oaklawn, Ill. Application July 18, 1956, Serial No. 598,582 4Claims. (Cl. 331-49) My invention relates to electronic organs andanalogous musical instruments, of the type in which the original tone isgenerated electronically, as by means of electrical oscillators, with orwithout additional distorting devices for changing the wave shape of thesignal generated by the oscillators. For use in'such instruments theoscillator, ideally, must have certain characteristics not generallyfound in ordinary oscillators.

Specifically, the frequency of the generated signal must beexceptionally stable over long periods of time, substantiallyindependent of aging of the vacuum discharge devices or transistors, andunaffected by variations in the potential of the power supply, over therange of voltage variations to which ordinary commercial power circuitsare liable.

Further, in cases where it is desired to have the oscillator normallyinoperative, and to key or play the instrument by applying activatingpotential to selected combinations of oscillators, a much more diflicultre- To secure-effective control of the rates of attack and decay when anote is played, it is essential that the amplitude of the signals.generated should correspond roughly to theactivating voltage deliveredto the oscillator, and that over substantially the entire range ofamplitudes there must be no material change in frequency. It isessential to secure this constant pitch over any desired period ofattack or decay because even a small change in pitch, especially duringdecay, produces a chirp that is most objectionable from the estheticpoint of View. Many percussion instruments are characterized by longdecay periods up to several seconds at perfectly constant pitch and theextended period of the decay makes this'requirement critical, whenpercussion effects are desired.

In designing oscillators for these exacting requirements of electronicmusical instruments, two different stability citeria must be carefullyconsidered. First, the primary elements of the tuned circuits, namely,the inductance and capacitance, must remain stable, not only with regardto aging, temperature, and humidity, but especially in regard toinstantaneous changes as a result of the normal functioning of theinstrument itself with varying excitation applied to the oscillatorcircuit, and changes in the currents flowing in the circuit.

Secondly, the Q of the circuit must be as high as possible to minimizethe frequency shift due to the exciting impulse itself. This means thatthe reactance of the tuned circuit elements must be high compared withtheir resistance. This is vitally necessary because .any resistance inthe tuned circuit, whether it is the inherent characteristic of theinductor or capacitor, or Whether reflected into the tuned circuit bythe exciting circuit, will cause phase shifts of thereactive currents,which will cause a frequency shift away from the theoretical frequencyduring functioning.

In every-tuned circuit, there is a mathematically precise theoreticalfrequency depending on the values of the component parts of the tunedcircuit proper. This freqnency never obtains in practice because itcould only exist momentarily with no excitation taking place. Thepresence of the exciting circuit introduces disturbing factors which cannever be eliminated but can be reduced to a minimum by designing thecircuit with as a high a Q as possible. This variation from theoreticalfrequency will ordinarily be a maximum when the excitation is maximum,and may decrease materially before the applied plate voltage gets lowenough so that oscillation ceases.

With the air wound coils of which I have knowledge, gradual decay downto the voltage where oscillation ceases appears to be accompanied by alowering in pitch or frequency too slight to be detected by an untrainedear and too slight to be olfensive in any event. Ferromagnetic. materialfor a minor part of the magnetic circuit has a tendency to cause thefrequency to rise during the decay. During decay, this counteracts theeflect of the elimination of the exciting circuit (which is in shuntwith part of the inductor Winding), as a factor tending to make theactual frequency higher than the mathematical, theoretical frequency ofoscillation without excitation. A practical limit is reached, where thenet tendency to rise during decay becomes excessive. With ferro-magneticmetal cores which are also of conductive material, the amount offerro-magnetic material that can be tolerated, while maintainingadequate stability, is very low. With the ceramic, insulating,ferro-magnetic material disclosed herein, it is possible to use muchgreater amounts of ferro-magnetic material and much smaller physicaldimensions and still maintain adequate stability during decay. I believethis to be because hysteresis, and the variations in the permeabilityand the incremental permeability of the ceramic material, as a functionof flux density, differ substantially from the correspondingcharacteristics of metallic ferro-magnetic materials. Even with theceramic material, it is still necessary to have maximum flux densitiesmuch lower than those customary in the ordinary uses of such material.This limits the operation to a relatively narrow range within which thepermeability is relatively constant and the incremental permeability hasa negative coefiicient.

A special combination of selected constituents and proportioning of theparts has been found to accomplish:

(1) minimum pitch fluctuation with varying flux density;

(2) minimum pitch fluctuation due to ohmic and/0r magnetic losses;

(3) a residual fluctuation with variation in flux density that tends tocounteract the variation due to the phase differences between thecurrents in the tank circuit, the plate circuit, and the grid circuit.

(4) small, compact units having small, compact magnetic fields, suchthat complete tone generator assemblies can operate without magneticinterference in space volumes amounting to only a small fraction of thevolumes previously required.

It is significant that the space requirements are not merely the actualvolumes occupied by the physical units themselves. They include also thevolumes of the spaces surrounding each individual inductor, in whichspaces the magnetic fields of the inductors may come into such proximitythat the fields of two adjacent inductors occupying adjacent positionsin space, tend to overlap and affect each other. When this happens, thesignal from each inductor will be transferred into the other inductor.

If this contamination were followed by mere amplification withoutfurther alteration of wave shape, the final result might still betolerable. But when the contaminated signal is subsequently subjected toamplitude distortion of its wave shape, the distortinginstrumentaliasaansa r ties generate not only overtones of thecontaminating signal, but oscillations having a frequency equal to thesum of the frequencies of the two signals and overtones of thatoscillation, and other oscillations having a frequency equal to thedifference between the frequencies of the main signal and thecontaminating signal, and overtones of that oscillation. Notinfrequently three different oscillators may be functioning next to eachother and if there is such leakage followed by such distortion, thenumber of possible combinations builds up to a point such that theproduct is likely to be noise rather than music.

Even where the degree of contamination is relatively slight, thesubsequent attempts to atler the wave shape and produce tones of greatesthetic beauty, as described in U.S. Patent 2,649,006, is certain togenerate so many of these irrelevant or ghost frequencies, as to renderthe music esthetically unacceptable.

In the accompanying drawings:

Figure 1 is a schematic wiring diagram of an electronic organ;

Figure 2 is a schematic wiring diagram of an oscillator employing avacuum tube;

Figure 3 is an elevation of the separated parts of an inductance unit;

Figure 4 is a top plan view of an inductance unit; Figure 5 is a sectionas on line 5--5 of Figure 4; Figure 6 is a diagram of flux distribution;

Figure 7 is a perspective of a chassis for producing each of twodifierent semitones in each of six octaves;

liigure 8 is a plan view of the chassis on a reduced sca e;

Figure 9 is an end view of the same chassis;

Figure 10 is a schematic wiring diagram of a transistor oscillator; and

Figure 11 is a side elevation, partly in section, of a modifiedinductor.

In the embodiment selected to illustrate the invention, and referringfirst to Figure 1, there are indicated three tone-generating units 10,12, and. 14. Each of these units may be according to Figure 2 or Figure10. Each oscillator delivers a wave of dependably constant frequency andsubstantially perfect sine curve shape. The complete organ includes asmany oscillators as there are notes within the gamut of the instrument.The signals from the oscillators pass through distorting devices 16 ofwhich six are illustrated for each oscillator. Distorted signals ofsimilar tone quality are collected on the buses 18. Stops 20 controlledby the player connect any selected assortment of buses to the amplifier22, with the resultant music delivered by the loud speaker 24. Thesedistorting units are fully described in Patent 2,649,006 August 18, 1953and per se form no part of the present invention.

The playing keys 26 are used for normal playing to activate selectedcombinations of oscillators. The playing keys 28 activate theoscillators with the same attack characteristics but with prolongeddecay characteristics to produce percussion effects. This isaccomplished by capacitors 30 protected by resistances 32 and separatedfrom the oscillators by rectifiers 34 so that keys 26 can operate theoscillators without charging the condensers but when keys 28 are usedthe capacitors 30 charge when the note starts and the charge isavailable to prolong the decay of the note when the key is opened.

Referring to Figure 2 the conductor 36 receives positive potential fromthe power source 38 when the player closes either key 26 or key 28. Theconnection to the tube 46 is through a time-delay circuit comprising theresistor 40 and grounded capacitor 42 to the plate 44 of the vacuum tube46. The cathode 48 is kept continuously active so that the oscillationis initiated by supplying potential to plate 44 and controlled duringthe function of the oscillator by the grid 50. The cathode and grid areconnected across only 30 percent of the winding 52 4 of the inductor 54.Thus the conductor 56 connects the cathode to an intermediate point inthe winding and the grid is connected through the protecting capacitor58 to the junction point 60, which is equivalent to the upper end of thecoil as illustrated in Figure 2. The theoretical frequency of the tunedloop comprising the inductance 54 and the capacitor 62 is a function ofdimensions of those elements only, but that frequency never exists inactuality because the exciting current has to supply an increment ofenergy during each oscillation to replace that dissipated in the loop.The dissipation of energy takes place due to the ohmic resistance of thewinding 52; and to hysteresis in the core of the inductance if it has acore. There are also dielectric losses in the capacitor 62, magneticleakage losses, and resistance losses in the connectors.

The intermittent supply of energy by the exciting circuit alters thefrequency and this alteration varies with the intensity of excitaiton.This variation cannot be avoided but it can be minimized by designingthe circuit with maximum Q.

Referring now to Figures 3, 4, 5, and 6 each mductance comprises acylindrical core 64 and an annular winding 66. The length of the coreand the outer diameter of the winding have a ratio approaching umty, saybetween 0.7 and 1.4.

A suitable structural frame is provided including the guide tube 68, inwhich the core may slide freely, and spaced end plates 70 and 72 for thecoil. The tube 68 extends beyond the plates in one direction and theopposite plate carries a plurailty of connector terminals 74, 76, and78.

Figure 6 is a diagram of the putative approxlmate distribution of thelines of magnetic flux when the energy storage in the inductance is atits peak. It will be noted that the outer envelope of the lines of forceis approximately spherical and that the radial dimension of the crosssectional area in which the lines of force in air are located can bevery small at the equator because the other dimension is the entireperiphery of the sphere. Accordingly, the lines of force at the pointindicated at in Figure 6 will all lie very close together in a thinannular band and if there were another inductance beside the oneillustrated, the spacing between them needed to prevent deleteriousintermingling of the lines of force of the two oscillators would be aminimum. For instance, the point 82 at the right side of Figure 6represents a degree of separation that would be efiiective.

Means are provided for shifting the axial position of the core 64 withrespect to the coil. This provides a very fine and precise adjustment ofthe inductance. I have illustrated a cap 84 telescoped over the tube 68and provided with small prongs 86 which embed themselves slightly in thematerial of the tube. The core 64 carries an integrally assemblednon-magnetic threaded extension 88 provided with a screw driver slot 90,and the cap 84 has tangs 92 engaging the threads of the adjustment screw88.

Referring now to Figures 7, 8, and 9, the geometrically compactconfiguration of the field indicated in Figure 6, in which field thepulsating energy is stored, makes it possible to locate a plurality ofinductances much closer to each other than was heretofore possible. Thecentral channel 94 of the chassis is of non-magnetic metal such as sheetaluminum. Twin panels 96 and 98 of fiber board are aflixed to the edgesof the channel legs. The panel 98 is apertured to receive sixinductances of which four are in spaced relation along a straight lineextending down the middle of the panel. The largest inductance 100,which produces the lowest note, is at the upper end of the panel 98 inFigure 7. The next inductance, musically, is the fourth inductancegeometrically, at 102, and is part of a loop tuned to produce afrequency twice that of the inductance 100. The inductance 104 isadjacent the inductance 100 and produces a note two octaves higher thanthe inductance 100. The next inductance 106 is at the extreme remote endof the panel 98 and offset toward the channel 94. It produces afrequency four octaves above that of inductance 100. The fifth octave isproduced by inductance 108 located on the center line between inductance102 and inductance 104. Finally the sixth octave is produced byinductance 110 which is at the extreme end of the panel, almost but notquite abreast of inductance 106 but set out to the outer corner of thepanel remote from the channel 94. As most clearly indicated in Figure 8,panel 96 has identical arrangement except that the largest inductancefor producing the lowest note is at the end of: panel 96 farthest fromthe inductance of panel 98. Also, when a plurality of chasses accordingto Figure 7, are set side by side, each inductor 100 is nearest theinductor 110 of the adjacent chassis.

It is customary to have each panel produce the same note of the musicalscale in each of six different octaves. For instance. panel 98 mightproduce the note C and panel 96 might produce the note C sharp. It willbe apparent that twelve panels assembled on six channels can be used toproduce 72 consecutive semitones. No inductor is adjacent anotherinductor of frequency closer to it than two full octaves, but the sixthoctave is so small that it can find room beside the fourth octave.

Along the web of the channel 94 are provided sockets 108 receiving twinvacuum tubes 110. Each such tube is provided with two plates and twogrids and constitutes the tube 46 for two oscillating circuits, one ofwhich will be on panel 96 and the other on panel 93.

A convenient and geometrically compact arrangement for the resistors andcapacitors indicated schematically in Figure 2 is a pair of fiber boardpanels 112 and 114 provided with terminal strips 116 each carrying aplurality of terminals 118 for establishing the connections called forin Figure 2 with the various resistors and capacitors indicatedgenerally at 120 in Figure 7.

By arranging the panels or Wings 112 and 114 in the diagonalrelationship shown, a compact arrangement of the complete assemblywithin the confines of a rectangular parallelepipedon of minimum volumeis obtained.

Referring now to Figure 10 I have indicated schematically a transistor122 with its emitter 124 connected to conductor 56 through an adjustableresistor 126. The base 128 is connected to the capacitor 58 and thecollector 128 is connected to the time-delay circuit through a resistor130. The gridbias resistor 132 of Figure 2 is omitted. A resistor 133 isconnected between the collector and the base and a resistor 134 isconnected between the base and ground. The three resistances 138, 133,and 134 constitute a three-element network which keeps the potentials ofthe collector and the base within pre-determined limits.

All other parts of the oscillating circuit illustrated in Figure 10perform the same function as in Figure 2 but the absolute values need tobe changed because the transistors illustrated operate with the powersource 38 in opposite polarity and at voltages between about 6 and about18 volts.

Audio frequency oscillators are commonly made with a plate circuitincluding from 10 to 30% of the tank circuit, the rest of the tankcircuit becoming part of the grid circuit. An oscillator according tothe invention employs an inductor of much less inductance and anabnormally large capacitor, such that the circuit does not oscillatewell with such a disposition of the tap connected to the cathode.Specifically, the inductor is so small and the capacitor so large thatoscillation achieves maximum intensity and stability with about 70% ofthe tank circuit in the plate circuit. Beginning with about 10% of thetank circuit in the plate circuit the response of the oscillator and itsstability both improve gradually up to optimum values over a shortplateau located at about 70% and then decrease abruptly. The smallinductor and large capacitor constitute an abnormal and relativelyinefficient combination, electrically speaking, but the high stabilityof the frequency of the entire combination transcends otherconsiderations.

Referring now to Figure 11 I have indicated an inductor in which theceramic core 138 has a cap 142 of the same ceramic ferro-magneticmaterial. The cap may have a peripheral lip 144 extending down aroundthe outside of the coil 146, which coil may occupy the entire enclosedspace except for the winding tube 148. A duplicate cap 144 results inreducing the air path to the relatively short gap at 148. The air pathis not only short in axial dimensions but of quite large cross sectionand if made up in the same size as in Figure 6 the air gaps at 148 andthrough the tube 68 would present only ten or twenty percent as muchresistance to the flux as the long air gap in Figure 6. Because of thehigh permeability of the core 138 and the caps 142 the air gap of Figure11 may still represent considerably more than percent of the totalresistance to flux, but the necessary total flux to obtained with astructure having dimensions less than half and volume less than A; ofwhat would be required to get the same frequency with an inductoraccording to Figure 6. For very low notes where percussion effects arenot desired this secures adequate stability with a great decrease insize.

Others may readily adapt the invention for use under various conditionsof service by of the novel features disclosed For instance, with thetransistor the channel 94 is omitted.

As at present advised with respect to the apparent scope of my inventionI desire to claim the following subject matter.

oscillator of Figure 10, structurally superfluous and may be eachtransistor having an emitter a collector and a base; each tank circuitcomprising an inductor and a capacitor connected into a closed loop;said inductor having a wound coil of many turns; said exciting circuithaving a connection from said base to one point in said quency down tocessation; celving and assembling into a composite signal, the signalsfrom a plurality of simultaneously oscillating oscillators.

2. A combination according to claim 1 including means for securing smallprecision pitch adjustments of the pitch of each oscillator by shiftingsaid ferro-magnetic core axially with respect to said coil.

3. A combination according to claim 1 in which said exciting circuitincludes about 70% of said inductor coil.

4. A combination according to claim 1 which six inductor coils aremounted in a common plane; there being a single panel for supportingsaid coils as a unitary assembly; each of said coils being part of oneof said oscillators; each oscillator being electrically separate fromall the others to the extent that it can oscillate independently of allthe others; the frequencies of five of said oscillators being aliquotportions of the frequency of the oscillator of highest frequency; thealiquot portions being V2, 34, /8, A the coil for the lowest frequency,identified for convenience as the first octave, being adjacent one endof said panel; the coils for octaves l, 3, 5

and 2 being arranged in a longitudinal line in the order stated; thecoils for octaves 4 and 6 being spaced apart transversely on oppositesides of said line and beyond the coil for octave 2.

References Cited in the file of this patent UNITED STATES PATENTS2,051,012 Schaper Aug. 11, 1936 2,216,513 Hammond Oct. 1, 1940 2,223,539Baker Dec. 3, 1940 2,291,787 Beanland et al. Aug. 4, 1942 2,555,039Bissonette May 29, 1951 2,588,082 Brown et al. Mar. 4, 1952 2,603,774Gusdorf et a1 July 15, 1952 2,630,560 Earl et a1. Mar. 3, 1953 2,728,054Schoenberg Dec. 20, 1955 2,790,906 Hammond Apr. 30, 1957 min...

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Pat ent No. 2,9247841 February 9. 1960 Richard H. Peterson It is hereby certified thaterror a of the above numbered patent re Patent should rea ppears in theprinted specification quiring correction and that the said Letters d ascorrected below.

Column 2, line 19 after "circuit" insert a comma column 5 line 14 forthe numeral "10" read 100 Signed and sealed this 26th day of July 1960(SEAL) fittest:

KARL H.- AXLINE ROBERT C. WATSON .ttesting Officer Commissioner ofPatents UNITED STATES PATENT OFFICE v CERTIFICATE OF CORRECTION PatentNo. 2,924,784 February 9, 1960 Richard H. Peterson It is herebycertified that error appears in the printed specification of the abovenumbered patent requiring correction and that the said Letters Patentshould read as corrected below.

Column 2, line 19, after "circuit" insert a comma; column 5, line ll forthe numeral "10" read lOO Signed and sealed this 26th day of July 1960.

(SEAL) ittesti KARL H.- AXLINE 7 ROBERT C. WATSON tttesting Ofi'icerCommissioner of Patents

